Cobalt as a Probe for Zinc in Metalloenzyme Model Compounds? A Comparison of Spectroscopic Features and Coordination Geometry of Four- and Five-Coordinate Complexes. Crystal and Molecular Structures of [Co(η3-TpPh)(η2-TpPh)], [(η3-TpPh)Zn(anthranilate)], and [(η3-TpPh)M(η2-acac)] (TpPh = Hydrotris(3-phenylpyrazol-1-yl)borate, acac = Pentane-2,4-dionate, and M = Zn, Co)

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Abstract
The reaction of cobalt(II) perchlorate with 1 equiv each of potassium hydroxide and potassium hydrotris(3-phenylpyrazolyl)borate (KTpPh) leads to the formation of the blood-red bis(ligand) complex [Co(η3-TpPh)(η2-TpPh)]. This compound crystallizes in the monoclinic space group P21/n, with a = 10.690(5) Å, b = 34.243(15) Å, c = 12.923(4) Å, β = 96.17(3)°, Z = 4, V = 4703(3) Å3, and R = 0.040. The molecular structure contains a square CoN5 pyramid with an agostic BH···Co interaction of 2.17(2) Å. One TpPh ligand acts tridentate; the other, bidentate. The reaction of KTpPh with zinc and cobalt halides yields a series of heteroleptic halogeno complexes [(TpPh)MX]. The zinc species are all tetrahedral, while in the case of cobalt(II) the UV−vis spectra indicate an equilibrium of tetra- and pentacoordinated species depending on the anion size and the donor properties of the solvent. Metathesis reactions with carboxylates yield mononuclear complexes [(TpPh)M(O2CR)]; M = Zn(II), Co(II); RCO2- = acetate, benzoate, 4-fluorobenzoate, and 4-nitrobenzoate. The infrared bands νas(CO2) and νsym(CO2) indicate monodentate carboxylate ligands in the zinc complexes in the solid state and in solution. The cobalt complexes [(TpPh)Co(carboxylate)] are dark blue in dichloromethane and in the solid state when grown from dichloromethane solution. Tetrahydrofuran solutions and crystals grown from tetrahydrofuran are pink, purple, or reddish violet. The 2-aminobenzoate (anthranilate) complexes [(ThPh)M(anthranilate)], M = Zn(II), Co(II), have been prepared. [(η3-TpPh)Zn(anthranilate)] crystallizes in the monoclinic space group P21/n, with a = 13.205(4) Å, b = 15.643(3) Å, c = 15.116(3) Å, β = 98.86(2)°, Z = 4, V = 3085(1) Å3, and R = 0.034. Anthranilate acts as a chelating oxygen ligand with Zn−O distances of 1.932(2) and 2.460(2) Å. The amino group is not involved in metal coordination. The reaction of the chloro complexes [(TpPh)MCl] with sodium or potassium acetylacetonate leads to isotypic zinc and cobalt complexes [(η3-TpPh)M(η2-acac)]. Both crystallize in the monoclinic space group P21/c, with Z = 4, the zinc compound with a = 16.334(3) Å, b = 11.545(2) Å, c = 15.816(3) Å, β = 94.77(1)°, V = 2972(1) Å3, and R = 0.039 and the cobalt compound with a = 16.358(7) Å, b = 11.519(2) Å, c = 15.847(6) Å, β = 95.30(3)°, V = 2973(2) Å3, and R = 0.035. The structures are best described as slightly distorted trigonal bipyramids with the two axial positions occupied by one of the acetylacetonate O and one of the tripodal N donor atoms. Ligand profiles of TpPh and related ligands have been calculated to visualize the angular encumbrance of the ligands.

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